The gain of a repeater is to a large extent determined by the amount of isolation that can be achieved between the donor and server antennas of the repeater (see diagram 100 of FIG. 1). The higher the isolation, the higher the potential gain of the repeater.
When the donor and server antennas are in the same physical enclosure, or at least in close proximity to each other, maximizing the isolation level becomes increasingly difficult. Typically, repeaters with integrated donor and server antennas utilize highly directive antennas on either the donor or the server side, or more commonly on both the donor and the server side to achieve high isolation. An example of a commonly used architecture is to use directive patch antennas on a large ground plane to provide high isolation levels. When the operating frequency of the repeater is high (around 2 GHz and higher), this is a reasonably good strategy as the wavelength of the signals is relatively short. However, at lower frequencies, such as the 3GPP bands 5 and 12 (700-900 MHz), the wavelength of the repeated signal is long (300-400 mm) which means that the physical size of a patch antenna and ground plane needs to be large to achieve high isolation.